Northward Shift and Narrowing of the ITCZ in 20 Years of AIRS Data

Abstract

AbstractA time series analysis of Deep Convective Clouds (DCC) in Atmospheric InfraRed Sounder data between 2002 and 2023 shows that the mean latitude of the Inter Tropical Convergence Zone (ITCZ) has shifted northward for land at a rate of about 30 km/decade. The shift for ocean is north as well at about half of the rate for land. Comparisons of our results with the existing literature are complicated by the use of different methodologies and data sources: We used the daily observations of DCCs, which by their association with extreme rain rates are indicators of locally extreme vertical velocity on 15 km horizontal scales, while most of the literature used model or reanalysis gridded data with typically 100 km bins. The southern boundary of the ITCZ shifts north faster than the northern boundary, as qualitatively stated in the literature. The resulting contraction of the ITCZ width with global warming was qualitatively predicted based on theoretical consideration. Our observation of the rate of decrease in the width for ocean is statistically consistent with the contraction deduced from European Centre for Medium Range Forecast Reanalysis and Modern‐Era Retrospective analysis for Research and Applications data. The observed northward shift of the ITCZ was not predicted and is not seen in climate models in the 21st century.